Just to add something in the mix which contradicts a lot of people saying they have removed ads in case they were causing panda problems. We added adsense, although I have been against it for years as ive always seen it as non user friendly, and saw an improvement in serps within a week - coincidence?!
People have said it in the past google is about making money. Perhaps panda is actually a content/adsense ratio or measurement between content and adsense looking for a score on quality content to please the user but ultimately convert the user to an adsense click instead of a backwards click. Panda could be an algo looking to monetise every click whilst maintaining searcher satisfaction - the have enough data to acheive this.
Consider two identical sites both with the same content etc but one displays adsense. Google knows which one makes them money and has data on the conversion rate so which one would it put first.....
It is changing the game, the sites which generate it the most money for them come first, they have monetised the organic listings.